1. Field of the Invention
The present invention relates to a transmission for driving left and right axles of a vehicle comprising a pair of hydrostatic transmissions (to be referred to as HSTs), each of which is a fluid combination of a hydraulic pump and a hydraulic motor, wherein one of the HSTs is provided for speed change in advancing and reversing and the other is for steering.
2. Related Art
U.S. Pat. No. 4,782,650, for example, describes a vehicle provided with a pair of HSTs disposed in a lateral row and connected with each other, with left and right axles projecting laterally outward from its respective HST. Driving wheels are fixed onto utmost ends of the axles. The left and right axles are driven by changing the angles of slant of movable swash plates provided for the its respective left or right HST.
The left and right axles are driven at the same speed by the pair of HSTs when the vehicle is advancing or reversing in a straight line. They are driven at different speeds when turning.
However, the above mentioned conventional vehicle can advance or reverse straight only when the output rotational speeds of the pair of the HSTs are equal. Thus, equalization of the output speed of each of the HSTs is required. This equalization takes a long time during operation of the vehicle. Additionally, accuracy is required when manufacturing and assembling the components of each HST. If there is any difference in volume of the hydraulic pumps or motors between the HSTs, the feeling in turning left and right are different from each other, thereby making the vehicle difficult to control.
Furthermore, a speed changing operating tool, such as a speed change pedal, is connected to both output speed changing members, such as a movable swash plates, of the pair of HSTs. A steering operating tool, such as a steering wheel, is also connected to both members. This arrangement requires complicated linkages interposed between the speed changing operating tool and the both speed changing members and between the steering operating tool and both speed changing members, thereby increasing the manufacturing cost of the linkages and the space for disposal thereof in the vehicle.
If one HST is provided for speed change in traveling and another is provided for steering, each of the operating tools for speed changing and steering need only be connected with one output speed changing member of either of the HSTs, thereby simplifying the linkages between the operating tools and the speed changing members and reducing the manufacturing cost and the space required for disposal thereof.
However, the transmission for speed changing and steering must be capable of turning along a small radius if it is employed by a working vehicle like a mower tractor required to travel among trees. It also must be compact if it is provided for a small vehicle.
Furthermore, it is required to be capable of being operated for speed changing and steering as easily and sensitively as a passenger car.
It is also desirable that the transmission's activity in turning corresponds to the traveling speed to improve the sensitivity of the steering operation. Moreover, it is desired for safety and efficiency of work that when the same degree of steering operation is employed, the vehicle provided with the transmission turns along a small radius when traveling slowly and turns along a large radius when traveling fast.